Organic materials such as natural polymers, synthetic polymers, oils and fats, lubricating oils or hydraulic fluids, lose their usefulness when oxidized. Therefore, various antioxidants have been devised to add to such organic materials. For example, it is known that hindered phenols, organic sulfur compounds, organic phosphorus compounds, and aromatic amines are effective antioxidants, which are used alone or in combination with each other. Examples of derivatives having the hindered phenol skeleton are disclosed in Japanese Patent Publication Nos. 2488/1958, 17164/1963, and 9651/1967. Those compounds are now practically used. Unexamined Published Japanese Patent Application Nos. 59835/1982, 128656/1982, and 128679/1982 disclose hindered phenol derivatives as the antioxidants. These conventional antioxidant compounds are not necessarily satisfactory under very severe conditions.